Unless otherwise indicated herein, the materials described herein are not prior art to the claims in the present application and are not admitted to be prior art by inclusion in this section.
A hollow cathode device is an electron source used in plasma devices. Within a hollow cathode device, there is an insert, from which electrons are thermionically emitted. One type of insert consists of tungsten that is partially filled with barium oxide, calcium oxide, and aluminum oxide among lesser constituents. Cathode inserts with barium oxide, calcium oxide, and aluminum oxide may be referred to as barium calcium aluminate cathode inserts. Some methods for making barium calcium aluminate cathode inserts involve numerous processing steps.
One type of hollow cathode insert consists of a porous tungsten matrix that is separately prepared to be 70-80 percent dense and 30-20 percent porous with an open pore structure. The porous cavities in the tungsten matrix are filled with a low work function barium-calcium-aluminate ceramic in a high temperature hydrogen furnace where the ceramic is liquefied and then pulled into the pores. The porous tungsten treated this way is referred to as having been impregnated with ceramic. In subsequent operation, a low work function occurs on the surface of the tungsten if a monolayer or partial monolayer of atomic barium and oxygen atoms is present on the tungsten surface. During operation, barium and barium oxide are constantly supplied to the surface of the insert from the pores via the open pore structure. The barium and barium oxide are produced at the pores through chemical reactions between the ceramic and its lesser constituents and with the ceramic and the tungsten walls of the pores. The barium and barium oxide constantly evaporate from the surface of the tungsten as a result of the elevated operating temperature, but the aforementioned constant supply of barium and barium oxide from the interior regions of the insert continuously renew the surface so that the work function remains low. The addition of a small amount of scandium oxide into the ceramic can help convert the barium-calcium-aluminate mixture contained within each pore into barium and barium oxide without the need for intimate contact and chemical reaction with the tungsten walls of the pores.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.